From: Jiufei Xue jiufei.xue@linux.alibaba.com Subject: fs/writeback.c: use rcu_barrier() to wait for inflight wb switches going into workqueue when umount

synchronize_rcu() didn't wait for call_rcu() callbacks, so inode wb switch may not go to the workqueue after synchronize_rcu(). Thus previous scheduled switches was not finished even flushing the workqueue, which will cause a NULL pointer dereferenced followed below.

VFS: Busy inodes after unmount of vdd. Self-destruct in 5 seconds. Have a nice day... BUG: unable to handle kernel NULL pointer dereference at 0000000000000278 [<ffffffff8126a303>] evict+0xb3/0x180 [<ffffffff8126a760>] iput+0x1b0/0x230 [<ffffffff8127c690>] inode_switch_wbs_work_fn+0x3c0/0x6a0 [<ffffffff810a5b2e>] worker_thread+0x4e/0x490 [<ffffffff810a5ae0>] ? process_one_work+0x410/0x410 [<ffffffff810ac056>] kthread+0xe6/0x100 [<ffffffff8173c199>] ret_from_fork+0x39/0x50

Replace the synchronize_rcu() call with a rcu_barrier() to wait for all pending callbacks to finish. And inc isw_nr_in_flight after call_rcu() in inode_switch_wbs() to make more sense.

Link: http://lkml.kernel.org/r/20190429024108.54150-1-jiufei.xue@linux.alibaba.com Signed-off-by: Jiufei Xue jiufei.xue@linux.alibaba.com Acked-by: Tejun Heo tj@kernel.org Suggested-by: Tejun Heo tj@kernel.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/fs-writeback.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-)

--- a/fs/fs-writeback.c~fs-writeback-use-rcu_barrier-to-wait-for-inflight-wb-switches-going-into-workqueue-when-umount +++ a/fs/fs-writeback.c @@ -523,8 +523,6 @@ static void inode_switch_wbs(struct inod

isw->inode = inode;

- atomic_inc(&isw_nr_in_flight); - /* * In addition to synchronizing among switchers, I_WB_SWITCH tells * the RCU protected stat update paths to grab the i_page @@ -532,6 +530,9 @@ static void inode_switch_wbs(struct inod * Let's continue after I_WB_SWITCH is guaranteed to be visible. */ call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn); + + atomic_inc(&isw_nr_in_flight); + goto out_unlock;

out_free: @@ -901,7 +902,11 @@ restart: void cgroup_writeback_umount(void) { if (atomic_read(&isw_nr_in_flight)) { - synchronize_rcu(); + /* + * Use rcu_barrier() to wait for all pending callbacks to + * ensure that all in-flight wb switches are in the workqueue. + */ + rcu_barrier(); flush_workqueue(isw_wq); } } _